> Hi Pete,  in your reading was there any discussion on what plant might prevail in the absence of knotweed. 

I have spent the past several weeks poring over the literature on knotweeds. It turns out not to be a particular problem in Japan. Here they describe the process of succession

> Reynoutria japonica is a common perennial pioneer species on Japanese volcanoes. In a volcanic desert (1500 m above sea level) on Mount Fuji (3776 m), central Japan, this species forms circular stands (patches). As a patch develops, shoot density decreases in its centre (‘central die-back’). The central die-back has been considered a key process in the early stages of primary succession, though its mechanism has been unknown.

Herbaceous species such as Miscanthus oligostachyus and Aster ageratoides occur in the central areas of these large R. japonica patches. Since these invading species are not observed in stands of other pioneer species or on bare ground, but only in the centres of large patches of R. japonica, central die-back of R. japonica patches probably facilitates primary succession on Mount Fuji.

* ADACHI, N., TERASHIMA, I., & TAKAHASHI, M. (1996). Central die-back of monoclonal stands of Reynoutria japonica in an early stage of primary succession on Mount Fuji. Annals of Botany, 77(5), 477-486.

* Shimoda, M., & Yamasaki, N. (2016). Fallopia japonica (Japanese Knotweed) in Japan: Why Is It Not a Pest for Japanese People?. In Vegetation structure and function at multiple spatial, temporal and conceptual scales (pp. 447-473). Springer, Cham.

comment

I have been examining the flowers of knotweeds for the past several weeks and have seen that most of them are males only, which accounts for why seed is so rare. The stands where I have seen seeds are all female plants. Evidently, pure Japanese knotweed is not common, that most of what I see is a hybrid. 

> The Japanese knotweed clone originally introduced was a male sterile female clone; thus, early in the invasion, reproduction from seed was not an issue. Widespread hybridization between Japanese knotweed and Sakhalin (giant) knotweed has been reported, with the hybrid species, Bohemian knotweed, forming the majority of knotweed plants in many areas and possessing higher variability than the parent species. 

> As early as 2007,, studies in North America indicated that the incidence of hybridization of Japanese knotweed and giant knotweed was far more prevalent than previously suspected. Genetic analysis in three Massachusetts knotweed populations by Grimsby et al. (2007) identified 26 genotypes. In a recent study by Gaskin et al. (2014) of populations in western North America, the hybrid Bohemian knotweed comprised a staggering 71% of plants sampled.

* Gillies, S., Clements, D. R., & Grenz, J. (2016). Knotweed (Fallopia spp.) invasion of North America utilizes hybridization, epigenetics, seed dispersal (unexpectedly), and an arsenal of physiological tactics. Invasive Plant Science and Management, 9(1), 71-80.

notes

Miscanthus oligostachyus = Flame Grass, Eulalia
Aster ageratoides = Ezo Murasaki Aster
Reynoutria japonica = Fallopia japonica = Polygonum cuspidatum = Japanese Knotweed
Male sterile female = hermaphrodite flowers with ovaries and non-functioning stamens

PLB

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